For reasons of decreasing volume, reducing weight, increasing power density, improving portability, and the like, three-dimensional (3D) semiconductor structures have been developed. Typically, stacks comprising a plurality of layers may be formed on the substrate, and separated from each other by high aspect ratio trenches. In some types of 3D semiconductor structures, doped portions extending in a vertical direction may be further arranged in the trenches. Such doped portions may be made by a formation process of vertically arranged polysilicon layers and a following (ion) implantation process. However, since the implantation process is typically conducted from the top of the whole structure and the portions are vertically arranged in the high aspect ratio trenches, it is difficult to obtain an uniform doping concentration in the vertical direction. Generally, a doping concentration near the top is higher than a doping concentration near the bottom. This condition may lead to electrical property differences between a device near the top and a device near the bottom.